Conventionally, TCP/IP has been well known as a communication protocol. In TCP/IP, an IP address is used as information to identify each device when communications between devices are performed. That is, when TCP/IP is used, uniquely determined IP addresses are allocated to devices on a network.
When each device transmits communication data to a network, the device transmits the data, attaching a transmission-destination IP address and a transmission-source IP address to the data. When communications are performed using plural networks, a communication relay device connecting the networks performs a path control using a transmission-destination IP address, and transfers communication data to a device that has the transmission-destination IP address.
When a local network is connected to the Internet, an IP address attached to communication data of a device on the local network is converted by a method called Network Address Translation (NAT). A NAT processing unit that performs NAT holds an IP address on the Internet, converts a transmission-source IP address, which is attached to communication data transmitted by a device on a local network and is received by the NAT processing unit, into an IP address on the Internet held by the NAT processing unit, and transmits after conversion the communication data to the Internet. The NAT processing unit also holds an IP address on a local network, converts a transmission-destination IP address, which is attached to communication data received from the Internet, into an IP address of a device on the local network, and transmits after conversion the communication data to the local network. In this way, a device on the local network can communicate with a device on the Internet via the NAT processing unit.
A technique of NAT can be used not only for communications between a local network and the Internet but also for communications between plural local networks having different network addresses (for example, see Nonpatent Literature 1).
Conventionally, to perform communications between devices arranged in vehicles of a train, there has been known a technique of performing communications via a communication relay device by arranging the communication relay device in each vehicle. For example, Patent Document 1 discloses a communication relay device that rewrites relative vehicle-formation numbers of a destination address and a transmission source address of a data communication packet. With this configuration, communications between arbitrary communication terminals installed in vehicles can be continued even when vehicle formations are merged.    Patent Document 1: Japanese Patent Application Laid-open No. 2004-201476    Nonpatent Literature 1: ‘Internetworking with TCP/IP’, by Douglas E. Comer, Vol. 1, 4th Edition, August 2002, KYORITSU SHUPPAN Co., Ltd.